First, a disclaimer that I don't actually think the number of PAK hours means much - how long a song PAKs purely depends on chart competition at the time and what other songs are releasing. Second, chart reforms in recent years have made maintaining a PAK much easier because the charts are a lot more stable. Only real-time Genie and Bugs will occasionally succumb to zombie/fandom streamers now. This is proven by how most of the longest-running PAK streaks are all songs from the past 2 years.
Currently 1st place for PAK hours is BTS "Dynamite" with 610 hours. IU's "Celebrity" is in 2nd place with 385 PAKs and still counting. If "Celebrity" streak continues mostly unchecked, it will overtake "Dynamite" in around 10 days (sometime between February 27-28).
Ironically, Celebrity's biggest threat is probably IU's next release. IU finally dropped a teaser for her upcoming 5th full-album today, but did not specify a comeback date. If IU doesn't release her album in the next 2 weeks, "Celebrity" has a chance of claiming the top spot.
Will "Celebrity" hang on for 10 more days? Will it be IU that stops her own streak? Or will an unexpected dark horse rise up on the charts in the next 1-2 weeks?
